Catalytically cracked naphtha (“cat naphtha”) boiling in the gasoline boiling range is generally high in octane number resulting from the olefinic species contained therein. Cat naphtha may also contain sulfur impurities in sufficient quantity to warrant removal by hydroprocesssing, for example, in order to comply with product specifications and environmental regulations.
While hydroprocessing is a common method for sulfur removal, it also results in saturation of the desirable olefin species leading to a loss in cat naphtha octane number.
Some conventional processes attempt to remove the sulfur while maintaining octane number by separating the naphtha into at least a light fraction and a heavy fraction. Such processes take advantage of the observation that the olefins are concentrated in the light fraction and the sulfur in the heavy fraction. Accordingly, the light fraction is not hydroprocessed to preserve the olefin content and may be subsequently combined with the hydroprocessed heavy fraction to provide a naphtha having less sulfur without too great an octane number reduction. Unfortunately, some sulfur, such as mercaptan and thiophene sulfur, is often present in the light fraction, so further sulfur removal from the naphtha would be desirable.
At least one conventional process attempts to overcome this difficulty by treating the light fraction with a cobalt group metal chelate catalyst in an alkaline medium to oxidize the mercaptans to disulfides which are separated from the light fraction. Even so, such a treatment would not remove the thiophene from the light fraction, and further sulfur removal would therefore be desirable.
In another conventional process, reactive separation is employed to provide a relatively sulfur-free light fraction and a heavy fraction that contains monosulfides (“converted monosulfides”) resulting from the reactive conversion. While the light fraction would then contain less sulfur, the hydroprocessing of the heavy fraction must be conducted at greater severity in order to remove the converted monosulfides, which leads to an undesirable loss in octane number from olefin saturation.
Yet another conventional process seeks to restore octane number in the hydroprocessed heavy fraction by subsequently cracking the hydroprocessed fraction with an acidic catalyst such as ZSM-5. While octane number may be improved, the amount of desirable olefin species in the heavy fraction will still be diminished. Moreover, in order to avoid poisoning the acidic catalyst, the hydroprocessing is generally conducted at high seventy to remove nitrogen impurities, leading to even more olefin saturation.
There is, therefore, a need for a process for forming a low-sulfur naphtha for gasoline blending from a cat naphtha without undesirable olefin saturation.